1. Field of the Invention
The present invention relates to an image sensor made in monolithic form for use in image shooting devices.
More specifically, the present invention relates to an image sensor formed of small elements.
2. Discussion of the Related Art
Conventionally, a monolithic image sensor comprises photodiodes and transistors formed in a silicon substrate. More specifically, each pixel of an image sensor comprises a photodiode associated with a transfer transistor having its drain region associated with a read and processing circuit comprising one or several transistors. An insulation trench extending from the substrate surface enables insulating the photodiodes from one another.
A concern of those skilled in the art is to decrease the size of image sensor pixels. For this purpose, it has been provided to decrease the electron detection and collection surface areas of the photodiodes. This requires increasing the thickness of the collection areas to be able to accumulate enough charges. This however raises several problems.
First, the size decrease of collection areas increases the risk of interferences, also called crosstalk, between the different photodiodes. Indeed, if the photodiodes are very close to one another, electrons intended to reach a given photodiode can be more easily attracted by a neighboring photodiode.
U.S. Pat. No. 7,239,003 provides, to limit the crosstalk, forming deeper insulation trenches. It can however be seen, especially in FIG. 6 of this patent, that the pixel dimensions are not optimized since a space 323 is necessary between the electron collection area of photodiode 326 and insulation trenches 328.
Further, when electrons are stored at the bottom of a thick area, they are difficult to transfer by means of a conventional transfer transistor formed at the substrate surface. Thereby, many electrons remain in the collection area after transfer. These electrons create remanence currents which appear on subsequent read operations and which disturb the photodiode detection.
There thus is a need for an image sensor comprising pixels of small dimensions and high storage capacities, with an insulation enabling limiting crosstalk phenomena and enabling full transfer of the electrons stored in a photodiode.